# GCP does not match altitude

**URL:** <https://community.pix4d.com/t/gcp-does-not-match-altitude/4348>\
**Category:** PIX4Dmapper Questions/Troubleshooting\
**Created:** [January 5, 2018, 6:26pm UTC](https://community.pix4d.com/t/gcp-does-not-match-altitude/4348 "2018-01-05T18:26:43Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![Patricia\_Sammia](https://avatars.discourse-cdn.com/v4/letter/p/97f17d/32.png) [@Patricia\_Sammia](https://community.pix4d.com/u/Patricia_Sammia)\
**Post date:** [January 5, 2018, 6:26pm UTC](https://community.pix4d.com/t/gcp-does-not-match-altitude/4348/1 "2018-01-05T18:26:43Z")

</div>

Hello folks. Can please someone help me? I am having problems with the GCP and Raycloud in step1. The project has 9 GCPs and only one is not macthing the altitude of the GCP and Raycloud. The others are fine.&nbsp;  
I already did reoptimization and still the same. The difference is about 9.2 metters. Please see the image. Thanks guys

&nbsp;

&nbsp;

---

<div class="post-metadata">

**Author:** ![Aaron\_Woods1](https://avatars.discourse-cdn.com/v4/letter/a/8797f3/32.png) [@Aaron\_Woods1](https://community.pix4d.com/u/Aaron_Woods1)\
**Post date:** [January 8, 2018, 4:29am UTC](https://community.pix4d.com/t/gcp-does-not-match-altitude/4348/2 "2018-01-08T04:29:42Z")

</div>

Hi Patricia,

According to the quality report, this project processed into 14 separate blocks. &nbsp;This is normally indicative of insufficient overlap among images. &nbsp;Corridor projects are also difficult due to their slender linear nature (requiring a higher number of GCPs per project. &nbsp;I would suggest 5 or more per block). &nbsp;Can you share with us the degree of overlap and flight plan you used for image capture? &nbsp;What type of GCPs are you using? &nbsp;The quality report indicates a mean RMS error (for 7 GCPs as 2.589 meters. &nbsp;Are you certain that you are accurately tagging the GCP locations &nbsp;within the GCP manager?

Best wishes,

Aaron